The invention relates to gamma-ray spectrometers, to systems employing gamma-ray spectrometers and to methods of gamma-ray spectroscopy.
Gamma-ray spectrometers are standard instruments used in a wide variety of scientific and industrial applications. Gamma-ray spectrometers are designed to absorb the energy of incident gamma rays and to convert the photon energy into an electronic signal proportional to the energy deposited in the detector. These instruments are used to quantify both the energy of the gamma rays produced by a source and their relative intensities. This information enables the user to determine the particular radioisotopes that are present and, for example, their relative concentrations. For many applications, it is important to be able to resolve gamma ray line-features even when they are grouped closely together. The other main performance characteristic is the stopping power of the spectrometer or its ability to absorb the gamma ray photons efficiently.
Two main types of gamma ray spectrometer are currently in use, hyper-pure germanium crystal spectrometers and scintillation spectrometers, each of which is now briefly described.
FIG. 1 of the accompanying drawings shows schematically a prior art hyper-pure germanium crystal (HPGe) spectrometer apparatus. The apparatus comprises a detector 1 housed in a detector capsule 2 attached to an arm 3 connected to a downwardly depending stick that is placed in a liquid nitrogen container 4. Germanium is a semiconductor having a relatively low band-gap and so has the advantage of generating one electron-hole pair, on average, for every 2.96 eV deposited in the crystal. This implies that for a full energy deposit of 1 MeV, the total number of charge carriers produced is of the order of 350,000. The statistical variance in such a signal is very small and so that this particular contribution to the achievable energy-resolution is typically only ˜0.5% FWHM.
One of the disadvantages of the HPGe detector is that it can only function as a spectrometer if cooled to liquid-nitrogen temperatures, otherwise electrons can be thermally excited into the conduction-band and so generate a high level of noise. This means that an HPGe detector is neither compact nor rugged. The second disadvantage is that in order to provide a stopping power equivalent to a commonly available size of scintillation spectrometer, the germanium crystal becomes very expensive to fabricate. Nevertheless, such detectors are very widely used because of their unmatched spectral resolution.
FIG. 2 shows an example of a spectrum acquired using a HPGe spectrometer (lower trace), in which energy E in keV is plotted against number of counts C.
FIG. 3 shows an example of a scintillation spectrometer. The detector comprises a scintillation crystal 31 which scintillates when a gamma-ray is absorbed within it. The scintillation crystal is fabricated from a material having a high effective atomic number. The scintillation crystal is packed into a hermetically sealed body 33 with Al2O3 which acts as a reflective material and receives signal through a glass entrance window 34 situated on the upper end of the package. The scintillation crystal is arranged above a photo-multiplier tube 32 which is encased in a magnetic shield 35. As an alternative to a photo-multiplier tube, a PIN diode can be used. PIN diodes are experimentally advantageous in terms of size and ruggedness, but have not been widely used except in small scintillation counters in which the use of bulkier photomultiplier is inappropriate. This is because PIN diodes have relatively small sensitive detection areas and relatively high noise levels. A common crystal material used for the scintillation crystal is NaI(Tl), i.e. sodium iodide doped with a trace of thallium. In this material, an energy-loss of 1 MeV will generate ˜38,000 photons in a narrow wavelength band centred on 415 nm. The sealed body 33 is needed, since NaI(Tl) material is very hygroscopic. In conventional scintillation spectrometers, these crystals are machined to form a cylinder having a diameter, for example 78 mm (3 inches) to match that of the photo-multiplier 32 as shown in the figure. The total dimensions of the detector package in the specific example are 82 mm maximum diameter and 146 mm height. The quantum-efficiency of photo-multipliers at the wavelength of interest, is typically 25% so the number of charge-carriers detected from a 1 MeV energy deposit is ˜8,000. This assumes a light-collection efficiency of 85% in transferring the scintillation light to the photo-cathode. The achievable spectral-resolution is clearly then much poorer than that of the HPGe sensor since the statistical contribution to the resolution is worse than 2.75% at 1 MeV.
FIG. 2 (upper trace) shows a typical energy-loss spectrum recorded using an industry-standard 78 mm (3 inch) NaI(Tl) spectrometer when illuminated by a Co-60 source. The contrast in performance with the HPGe detector (lower trace) is very clear.
The energy-resolution of a scintillation spectrometer is significantly worse than that predicted by photon-statistics alone. The additional degrading effects are a consequence of several factors. A first effect is the variance in the scintillation efficiency of the crystal itself, which is energy dependent and cannot be corrected for simply. A second effect is the non-uniformity of the response of the photo-cathode. A third effect is the variance in the light-collection efficiency of the crystal and photo-multiplier assembly for events that occur in different locations within the detector crystal.
For any given field of use, the performance limitations of scintillation spectrometers need to be compared with the experimental limitations of HPGe spectrometers. Clearly, a gamma-ray spectrometer combining the advantages of both types of prior art device would be highly desirable. For example, if one could improve the energy-resolution of scintillation spectrometers, there would be many applications of these devices for which there would otherwise be no other alternative but to use the more expensive and fragile HPGe spectrometers.
One route already successfully exploited to improve the energy-resolution of scintillation spectrometers is to apply spectral deconvolution to the raw energy-loss data collected by the spectrometer. Deconvolution is a well known technique used in spectroscopy and other diverse fields, in which a raw data spectrum obtained with a detection system is deconvolved with a response function representing the response of the detection system to known input signals.
Since deconvolution is based on computing the incident spectrum from the energy-loss data and the detector energy-response function, its success is dependent on accurately defining the detector energy-response function, which is not a trivial problem.
Generally, the observed spectrum O(E) can be represented by the integral:                               O          ⁡                      (            E            )                          =                              ∫            0            ∞                    ⁢                                    R              ⁡                              (                                  E                  ,                                      E                    0                                                  )                                      ·                          I              ⁡                              (                                  E                  0                                )                                      ·                                                   ⁢                          ⅆ                              E                0                                                                        (        1        )            
where the I(E) is the incident spectrum, and R(E, E0) is the detector response function. This equation can be discretised as:                               [                                                                      O                  1                                                                                    .                                                                    .                                                                                      O                  m                                                              ]                =                                            [                                                                                                                  R                        11                                            ⁢                                              R                        12                                                                                                  …                                                                              R                                              1                        ⁢                        n                                                                                                                                                        .                                                                                                                                                 …                                                        .                                                                                                              .                                                                                                                                                 …                                                        .                                                                                                                                      R                        m1                                            ⁢                                              R                        m2                                                                                                  …                                                                              R                      mn                                                                                  ]                        ×                          [                                                                                          I                      1                                                                                                            .                                                                                        .                                                                                                              I                      n                                                                                  ]                                +                      [                                                                                ɛ                    1                                                                                                .                                                                              .                                                                                                  ɛ                    m                                                                        ]                                              (        2        )            
Here the term εi(i=l, . . . ,m) is the noise contribution and Rij is the probability that an incident gamma-ray, having an energy falling into bin i, will be detected in bin j. An important task has been to identify the most appropriate method to use in order to predict the incident spectra, given the observed energy-loss spectra and the errors on those spectra.
The response matrix R for a standard 78 mm (3 inch) NaI(Tl) detector can be predicted, in part, by computing how the gamma ray photons interact in the particular scintillation material and for the particular dimensions selected for that spectrometer. This may be accomplished using, for example, the GEANT code developed by CERN.
This response of such a detector was calculated at 3 keV energy intervals over the range from zero to 3072 keV assuming that the gamma-ray source was located at a point 10 cm on-axis, above the top of the detector. An extra programme was then used to represent the broadening of the spectrum expected as a consequence of the optical photon-statistics, based on the known light-yield of the material chosen. A curve may then be fitted to the experimental data in order to represent the way that the energy-resolution varies as a function of the incident photon energy. The effectiveness of the various mathematical methods that are available for use in solving this inverse problem have been reviewed in reference [1].
FIG. 4A shows, as an example, a raw energy-loss spectrum collected with a standard 78 mm (3 inch) NaI(Tl) detector illuminated by a Ra-226 source, the graph plotting counts C against energy E in keV. FIG. 4B is the equivalent graph of the deconvolved result. Not only has the spectral-resolution been markedly improved by the deconvolution, but the accurate repositioning of the Compton-scattered events back into the full-energy peak has resulted in an improvement of the detector sensitivity by a factor of about four at 1 MeV.
Although spectral deconvolution techniques are clearly very successful in improving the results taken with scintillation detectors, the achievable improvement is limited and the quality of data taken with HPGe detectors is still far superior.